Insulating fiber linings sprayed-over refractories have been described at pages 42-44 of "Industrial Heating," November, 1984 issue. This reference discloses a sprayed-on thermally insulating fiber lining composed of a mixture of alumina-silica fibers and an inorganic binder of composition not disclosed in this article. A later publication by the same company (Manville Service Company) indicates the composition of a suitable binder to be acid aluminum phosphate.
A commercially available ceramic fiber binder is sold under the name "Strataseal.TM." by Strataseal Corporation, McMurray, Pa.. While the specific composition of this binder has not been revealed, this material is known to be toxic and highly acidic. When used, precautions must be taken to avoid inhalation of its fumes or fibers coated therewith or contact of it or coated fibers while wet with the skin. A further disadvantage of this system is revealed subsequent to its application to a steel furnace substrate. With passage of time, corrosion of the metal substrate has been observed, which corrosion is believed due to the acidic character of the this binder system which is believed to contain, inter alia, phosphoric acid.
A further problem with known spray-on ceramic fiber insulation systems is a lower level of wet adhesion and wet cohesion than is desired and/or necessary in many situations. The term "adhesion" as used herein is in reference to adherence of the ceramic fibers of a lining to a substrate such as a furnace surface. The term "cohesion" as used herein is in reference to coherence of the ceramic fibers of a lining to one another. The terms "wet adhesion" and "wet cohesion" as used herein are in reference to adherence and coherence respectively of the ceramic fibers while the vehicle of the binder system is present on the fibers. As the lining thickness builds up, its weight does also. When applied to ceiling surfaces, the weight of the newly installed wet lining commonly causes separation of the newly applied lining from its substrate. While this problem can be ameliorated by mechanical devices which are affixed to the substrate prior to application of the sprayable lining, such practice is not preferred due to additional materials and costs and reduction in thermal insulation efficiency. Such mechanical supports means may include the installation of metal mesh in spaced relationship to ceiling or overhead surfaces, or stud anchors which are affixed to the substrate and protrude in the direction of the lining thickness away from the surface of the substrate.
It is thus a primary object of the present invention to reduce or eliminate toxicity hazard associated with prior sprayed-on ceramic fiber insulation systems. It is also an object of the present invention to improve the wet adhesion and wet cohesion of the newly sprayed-on layer so that greater thicknesses of lining may be applied to an overhead surface with minimum need of mechanical support means. It is also an object of an embodiment of the invention to provide an inorganic-organic binder which facilitates handling and spraying of the ceramic fiber and binder.